1. Field of the Invention
The present invention relates to an optical amplifier that amplifies a signal light based on Raman amplification function, an optical amplifying repeater and an optical transmission apparatus that are constituted by using the optical amplifier. More particularly, the present invention relates to an optical amplifier that collectively amplifies a signal light, in which signal lights having a plurality of wavelengths different from each other are wavelength division multiplexed (hereinafter referred to as a WDM signal light), and an optical amplifier that comprises a flattening function of a gain for each signal light.
2. Description of Related Art
It is known that Raman amplification function is generated by allowing a pumping light having a specified wavelength to be incident on a signal light that propagates in an optical fiber transmission path, and thereby an optical amplification of the signal light can be performed (for example, refer to Japanese Patent Laid-open No.10 (1998)-022931 gazette). The optical amplification by Raman amplification is performed by allowing the pumping light having the specified wavelength to be incident on the optical fiber transmission path in an opposite direction to a progress direction of the signal light that propagates in the optical fiber transmission path.
Incidentally, the optical amplification utilizing Raman amplification function could be utilized independently. However, in most cases, it is normally used in combination with the optical amplification which has a rare-earth element doped optical fiber as an amplification medium (hereinafter referred to as a normal optical amplification). In an optical amplifying repeater arranged midway or at an output end of the optical fiber transmission path, and in a front optical amplifier arranged within an optical reception terminal station, the pumping light for performing Raman amplification is incorporated in addition to the normal optical amplification in order to improve the gain.
However, when the optical amplification is performed on the WDM signal light by Raman amplification function, a difference (deviation) occurs in optical output levels of each signal light included in the WDM signal light after the optical amplification, since Raman amplification itself has a wavelength dependency of the gain. With regard to the difference of the optical output level, amounts of the difference are added in the multi-step optical amplification and repeating transmission system, resulting in large influences to a transmission characteristic.
Generally, an output wavelength characteristic of the optical amplifier as described above is designed in a state such that the WDM signal light to be input does not have the deviation in the optical output level depending on a wavelength. However, the WDM signal light, on which the optical amplification was performed by Raman amplification function generated in the optical fiber transmission path connected to an input portion of the optical amplifier, further undergoes the normal optical amplification. Therefore, at the time when the WDM signal light is input to the optical amplifier, when the deviation exists in optical input levels among the signal lights included in the WDM signal light, the deviation is further increased by the normal optical amplification. As a result, a desired output wavelength characteristic cannot be obtained.
The gain by Raman amplification is different from each other depending on the wavelength of the signal light and having the wavelength dependency, and the wavelength dependency itself is dependent on the wavelength of a pumping light source. Accordingly, there is a problem of difficulties in controlling the wavelength characteristic of the signal light uniformly, due to scattering of the wavelength at the time of manufacturing the pumping light source, a wavelength fluctuation according to difference in a pumping light intensity and a temperature at which the pumping light source is used, and the like.
The object of the present invention is to maintain uniformity in the gain even when the optical amplification is performed on the WDM signal light by the optical amplification utilizing Raman amplification function. Moreover, another object of the present invention is to make the optical output level of each signal light included in the WDM signal light to be uniform.
The object of the optical transmission apparatus of the present invention is, to perform a stable optical transmission such that a large deviation does not occur in the optical output level of each signal light, even in the case where the WDM signal light is transmitted.
To solve the foregoing problems, the optical amplifier of the present invention comprises: a first pumping light source for Raman amplification, which outputs a first pumping light having a first wavelength; a second pumping light source for Raman amplification, which outputs a second pumping light having a second wavelength; and a wavelength multiplexer for Raman amplification, which allows the first and second pumping lights to be incident on the optical fiber transmission path where the signal light propagates. The signal light is Raman amplified in the optical fiber transmission path by the first and second pumping lights. A propagation direction of the first and second pumping lights may be backward or may be forward (the same direction) to that of the signal lights.
Although the signal light may be applied to a single signal light, it is, specifically, the WDM signal light, in which the signal lights having a plurality of wavelengths different from each other are wave length division multiplexed. In other words, the optical amplifier of the present invention, with regard to the pumping light for performing Raman amplification, allows two pumping lights having the wavelengths different from each other to be incident on the optical transmission path.
In the above-described constitution, to reduce the influence on the WDM signal light from the wavelength dependency of the gain, which occurs when the optical amplification by Raman amplification is performed, the first wavelength is the wavelength to allow the signal light to be Raman amplified such that the wavelength having the peak gain by Raman amplification becomes shorter than the wavelength of the signal light included in the WDM signal light. On the other hand, the second wavelength is the wavelength to allow the signal light to be Raman amplified such that the wavelength having the peak gain by Raman amplification becomes longer than the wavelength of the signal light included in the WDM signal light.
Herein, a gradient showing the relation between the wavelength and the gain in Raman amplification by the first pumping light and a gradient showing the relation between the wavelength and the gain in Raman amplification by the second pumping light are in opposite directions from each other. The optical output level and the wavelength of the first and second pumping lights are respectively set such that the gains for signal lights included in the WDM signal light, on which Raman amplification was performed by the first and second pumping lights, become substantially equal to each other.
In the above-described constitution, the first and second pumping light sources for Raman amplification can be constituted of: first and second laser diodes that oscillate first and second lights respectively; and first and second fiber gratings, which are arranged in a previous step of the first and second laser diodes, that selectively transmit the lights having the first and second wavelengths in a specified ratio and reflect residual light. Alternatively, the first and second pumping light sources for Raman amplification can be also constituted of: the first and second laser diodes that oscillate the first and second lights respectively; and first and second optical filters, which are arranged in a previous step of the first and second laser diodes, that selectively transmit the lights having the first and second wavelengths.
The wavelength multiplexer in the optical amplifier for Raman amplification of the present invention, more particularly, can be constituted of: first and second pumping light multiplexers for Raman amplification, which are arranged by connecting to an optical fiber connected to the optical fiber transmission path, that transmit the signal light and allow the first and second pumping lights to be incident on the optical fiber. Alternatively, the wavelength multiplexer can be constituted of: a pumping light multiplexer for Raman amplification, which multiplexes the first and second pumping lights and outputs a multiplexed pumping light; and a pumping light multiplexer for Raman amplification, which is arranged by connecting to the optical fiber connected to the optical fiber transmission path, that transmits the signal light and allows the multiplexed pumping light to be incident on the optical fiber. Moreover, the wavelength multiplexer can be also constituted of: a polarization combiner for Raman amplification; and a pumping light multiplexer for Raman amplification. The former performs the polarization combination of the first and second pumping lights and outputs the multiplexed pumping light, where the first and second pumping lights, which are respectively output from the first and second pumping light sources for Raman amplification, are perpendicular to each other, that is, in a polarization state. The latter, which is arranged by connecting to the optical fiber connected to the optical fiber transmission path, transmits the signal light and allows the multiplexed pumping light to be incident on the optical fiber.
For the purpose of improving uniformity of the gain of the signal light included in the WDM signal light or the optical output level itself, a constitution can be adopted where a feedback control is performed to the optical amplification by Raman amplification function, which is obtained by the above-described constitution. Specifically, in addition to the above-described constitution, the wavelength multiplexer is further allowed to comprise: an optical splitter for splitting a part of the WDM signal light and outputs the split WDM signal light; signal light extractor to extract two signal lights, which have the wavelengths different from each other, from each signal light included in the split WDM signal light; an optical receiver to detect the optical output level of each of the two signal lights extracted by the signal light extractor; and a pumping light output control circuit for controlling the optical output levels or the wavelengths of the first and second pumping lights for Raman amplification according to the amount of difference between the two optical output levels.
Although the above-described optical amplifier of the present invention has an independent optical amplification function only by Raman amplification function, it can be also used in combination with the normal optical amplification function, in which the optical amplification is performed by using the rare-earth element doped optical fiber (for example, an erbium doped fiber) as the amplification medium. Specifically, in the above-described optical amplifier, the optical amplification by Raman amplification and the normal optical amplification are used in combination by further comprising: an amplification medium for amplifying the signal light; a third pumping light source for outputting a third pumping light to allow the amplification medium to be in an excited state; and a wavelength multiplexer for allowing the third pumping light to be incident on the amplification medium.
The wavelength multiplexer in such a constitution may be arranged at a position from which the signal light is input to the amplification medium, and the third pumping light may be allowed to be incident on the amplification medium in the same direction as the propagation direction of the signal light. Alternatively, the wavelength multiplexer may be arranged at an output side of the amplification medium from which the signal light is output, and the third pumping light is allowed to be incident on the amplification medium in the opposite direction to the propagation direction of the signal light. On the side to which the signal light of the wavelength multiplexer for Raman amplification is output, an optical isolator may be arranged, which selectively transmits only the light propagating to the same direction as that of the signal light and obstructs the light propagating in the opposite direction. According to such constitution, a reflected returning light or the influence by the pumping light for Raman amplification, which comes from the optical amplifier arranged in a subsequent step, is reduced, thus more stable optical amplification can be performed.
The constitution of the feedback control for the purpose of obtaining uniformity of the gain of the signal light included in the WDM signal light or of the optical output level itself can be also applied to the constitution where the normal optical amplification is used in combination. Specifically, in addition to the above-described constitution, the optical amplifier is further allowed to comprise: the optical splitter for splitting a part of the WDM signal light and outputs the split WDM signal light; the signal light extractor to extract two signal lights, which have the wavelengths different from each other, from each signal light included in the split WDM signal light; the optical receiver for detecting the optical output level of each of the two signal lights extracted by the signal light extractor; and the pumping light output control circuit for controlling the optical output levels or the wavelengths of the first and second pumping lights for Raman amplification according to the amount of difference between the two optical output levels.
The optical splitter may be arranged at a position from which the WDM signal light is input to the wavelength multiplexer for Raman amplification. Alternatively, the optical splitter may be arranged at a position to which the WDM signal light is output from the wavelength multiplexer for Raman amplification. In the case of the former constitution, the feedback control is performed intended for the flatness of the WDM signal light immediate before it is input to the optical amplifier. On the other hand, in the case of the latter constitution, the feedback control is performed intended for the uniformity of the WDM signal light after the optical amplification.
The optical amplifying repeaters of the present invention are arranged midway of upper and lower lines, and perform optical repeating and amplification for the signal lights propagating in each direction. As a basic constitution, the above-described optical amplifying repeaters are constituted to be arranged in each of the upper and lower lines. Specifically, the optical amplifying repeater is the one that comprises: the optical amplifier for a down signal, which is arranged in the optical transmission path for the down signal; and the optical amplifier for an up signal, which is arranged in the optical transmission path for the up signal. The optical amplifiers for down and up signals comprise the optical amplifiers of the present invention as described above.
In the above-described constitution, two of the pumping light sources for Raman amplification may be provided for each of the upper and lower lines. For more efficient constitution, the first pumping light source for Raman amplification, which is provided in the optical amplifier for the down signal and the first pumping light source for Raman amplification, which is provided in the optical amplifier for the up signal use a common first pumping light source for Raman amplification. And, the second pumping light source for Raman amplification, which is provided in the optical amplifier for the down signal and the second pumping light source for Raman amplification, which is provided in the optical amplifier for the up signal use a common second pumping light source for Raman amplification. The first and second pumping lights for Raman amplification output from the common first and second pumping light sources for Raman amplification may be coupled and split by an optical coupler, and then the split lights may be respectively output to the wavelength multiplexer for Raman amplification, which is provided in the optical amplifier for the down signal and to the wavelength multiplexer for Raman amplification, which is provided in the optical amplifier for the up signal. Such a constitution can be adopted in the case of the optical amplification performed independently by Raman amplification as well as in the case where the normal optical amplification is used in combination.
In the constitution where the normal optical amplification is used in combination, the optical amplifiers for the down and up signals further respectively comprise fourth pumping light sources for outputting a fourth pumping light to allow the amplification medium to be in the excited state. The third pumping light source provided in the optical amplifier for the down signal and the third pumping light source provided in the optical amplifier for the up signal are made to be a common third pumping light source. The fourth pumping light source provided in the optical amplifier for the down signal and the fourth pumping light source provided in the optical amplifier for the up signal are made to be a common fourth pumping light source. The third pumping light output from the common third pumping light source and the fourth pumping light output from the common fourth pumping light source are coupled and split by an optical splitting coupler, and the split lights may be respectively output to the wavelength multiplexers provided in the optical amplifiers for the down and up signals.
The WDM signal light transmission apparatus of the present invention comprises: an optical transmission terminal station for wavelength division multiplexing a plurality of signal lights having the wavelengths different from each other and for sending out the WDM signal light; the optical fiber transmission path for propagating the WDM signal light; an optical receiving terminal station for receiving the WDM signal light; and the optical amplifier of the present invention, which is arranged midway of the optical transmission path to repeat and amplify the WDM signal light.
Similarly, the WDM signal light transmission apparatus, which has the upper and lower lines, for transmitting the WDM signal light in both directions comprises: the optical transmission terminal station for wavelength division multiplexing a plurality of down signal lights having the wavelengths different from each other and for sending out a down WDM signal light; the optical fiber transmission path for the down signal light for propagating the down WDM signal light; the optical receiving terminal station for the down signal light for receiving the down WDM signal light; the optical transmission terminal station for wavelength division multiplexing a plurality of up signal lights having the wavelengths different from each other and for sending out an up WDM signal light,; the optical fiber transmission path for the up signal light for propagating the up WDM signal light; the optical receiving terminal station for the up signal light for receiving the up WDM signal light; and the optical amplifiers of the present invention, which are arranged midway of the up and down optical transmission paths to repeat and amplify the up and down WDM signal lights.